<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script src="../js/jquery-3.3.1.min.js"></script>
    <style>
        .one {
            width: 100px;
            height: 100px;
            background-color: pink;
        }
        
        .two {
            background-color: blue;
        }
    </style>
</head>

<body>
    <div class="one"></div>
    <script>
        // var one = document.querySelector('div');
        // one.className = "two";
        $(function() {
            $(".one").addClass("two");
        })
    </script>
</body>

</html>